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Enzymatic Route to Chiral, Nonracemic cis-2,6- and cis,cis-2,4,6-Substituted Piperidines. Synthesis of (+)-Dihydropinidine and Dendrobate Alkaloid (+)-241D Robert Cheˆnevert* and Michael Dickman De´ partement de chimie, Faculte´ des sciences et de ge´ nie, Universite´ Laval, Que´ bec, QC G1K 7P4, Canada Received November 3, 1995X
Piperidine-based compounds are an important class of natural alkaloids found in plants, insects, and amphibians. A general asymmetric synthesis of 2-alkyl-6-methylpiperidines is presented via the enzymatic desymmetrization of meso cis-2,6- and cis,cis-2,4,6-substituted piperidines with Aspergillus niger lipase (ANL). The enzymatic reaction proceeds in excellent chemical yield and high enantiotopic selectivity (ee g 98%). The general method is used to effect the synthesis of (+)-dihydropinidine-HCl as well as the first asymmetric synthesis of dendrobate alkaloid (+)241D. Piperidine compounds constitute an important class of naturally occurring alkaloids found in plants and insects.1 In addition, a handful of monocyclic, piperidine-based alkaloids as well as numerous bicyclic ones have been identified in dendrobate frogs.2 What is remarkable about these alkaloids is the preponderance of 2,6-disubstitution, many times in the cis configuration, on the piperidine ring. Pinidine (1) is a well-known cis-piperidine compound found in several species of the family Pinaceae.3 An often associated compound, dihydropinidine (2) has recently been isolated from the Mexican bean beetle, Epilachna varivestis, as a minor constituent.4 Many cis-2,6-piperidine alkaloids 3 have been found in the venom of fire ants of the genus Solenopsis.5 The four monocyclic piperidines isolated from the skins of poisondart, dendrobate frogs all contain 2,6-disubstitution. The most well-known of these, cis,cis-2-n-nonyl-6-methyl-4hydroxypiperidine 241D (4), has recently been synthesized in its racemic form.6 Among the many bicyclic alkaloids containing a piperidine ring, the indolizidine, monomorine I (5), and decahydroquinoline cis-195A (6) are representative2c,7 (Figure 1). The interest in these compounds is well displayed by the wealth of published material detailing their sources, biological activities, and syntheses. Pinidine has been shown to be a powerful teratogen.3 Numerous studies have outlined the wide range of activities (necrotoxic, hemolytic, phytotoxic, insecticidal, antibacterial, and antifungal) that the 2-alkyl-6-methylpiperidines of fire ant venom possess.1b Racemic piperidine 241D (4) has Abstract published in Advance ACS Abstracts, May 1, 1996. (1) (a) Strunz, G. M.; Findlay, J. A. Pyridine and Piperidine Alkaloids. In The Alkaloids; Brossi, A., Ed.; Academic Press: New York, 1985; Vol. 26, pp 89-174. (b) Numata, A.; Ibuka, I. Alkaloids from Ants and Other Insects. In The Alkaloids; Brossi, A., Ed.; Academic Press: New York, 1987; Vol. 31, pp 193-315. (2) (a) Daly, J. W.; Edwards, M. W.; Myers, C. W. J. Nat. Prod. 1988, 51, 1188. (b) Daly, J. W.; Myers, C. W.; Whittaker, N. Toxicon. 1987, 25, 1023. (c) Daly, J. W.; Garraffo, H. M.; Spande, T. F. Amphibian Alkaloids. In The Alkaloids; Cordell, G. A., Ed.; Academic Press: New York, 1993; Vol. 43, pp 185-298. (3) Stermitz, F. R.; Tawara, J. N.; Biokhin, A.; Foderaro, T. A.; Hope, H. J. Org. Chem. 1993, 58, 4813. (4) Attygalle, A. B.; Xu, S. C.; McCormick, K. D.; Meinwald, J.; Blankespoor, C. L.; Eisner, T. Tetrahedron 1993, 49, 9333. (5) Blum, M. S.; Jones, T. H.; Fales, H. S. Tetrahedron 1982, 38, 1949. (6) Daly, J. W.; Edwards, M. W.; Garraffo, H. M. Synthesis 1994, 1167. (7) Takahata, H.; Momose, T. Simple Indolizidine Alkaloids. In The Alkaloids; Cordell, G. A., Ed.; Academic Press: New York, 1993; Vol. 44, pp 189-256. X
S0022-3263(95)01956-6 CCC: $12.00
Figure 1.
been found to block the action of acetylcholine by a noncompetitive blockade of the nicotinic receptor-channel complex as well as blocking the binding of [3H]perhydrohistrionicotoxin.8 The bicyclic indolizidines and decahydroquinolines show extensive physiological activities.2c,7 These alkaloids have been the object of intensive synthetic efforts resulting in a variety of racemic and asymmetric syntheses.1b,6,7,9 We report here the first total, asymmetric synthesis of (+)-241D (4). Building on previous work,10 the enzymatic desymmetrization of meso cis-2,6-disubstituted piperidines with Aspergillus niger lipase (ANL) was extended to meso cis,cis-2,4,6-trisubstituted piperidines, thus opening the way to the synthesis of enantiopure (+)-piperidine 241D ((+)-4). In addition, the synthesis of (+)-dihydropinidine hydrochloride ((+)-2-HCl) by the same chemoenzymatic method displays the generality of this technique. (8) Daly, J. W.; Nishizawa, Y.; Edwards, M. W.; Waters, J. A.; Aronstam, R. S. Neurochemical Res. 1991, 16, 489. (9) (a) Theodorakis, E.; Royer, J.; Husson, H. P. Synthetic Commun. 1991, 21, 521. (b) Comins, D. L.; Weglarz, M. A. J. Org. Chem. 1991, 56, 2506. (c) Higashiyama, K.; Nakahata, K.; Takahashi, H. Heterocycles 1992, 33, 17. (d) Takahata, H.; Bandoh, H.; Hanayama, M.; Momose, T. Tetrahedron: Asymm. 1992, 3, 607. (e) Lu, Z. H.; Zhou, W. S. J. Chem. Soc., Perkin Trans. 1 1993, 593. (f) Comins, D. L.; Dehghani, A. J. Chem. Soc., Chem. Commun. 1993, 1838. (g) Meyers, A. I.; Munchhof, M. J. J. Am. Chem. Soc. 1995, 117, 5399. (10) Cheˆnevert, R.; Dickman, M. Tetrahedron: Asymm. 1992, 3, 1021.
© 1996 American Chemical Society
Synthesis of (+)-Dihydropinidine Scheme 1a
a Reaction conditions: (a) MeOH, 2,2-dimethoxypropane, 12 N HCl; (b) (i) H2, (ii) K2CO3; (c) BnO2CCl or EtO2CCl; (d) LiBH4; (e) Ac2O.
Scheme 2a
a Reaction conditions: (a) MeOH, 2,2-dimethoxypropane, 12 N HCl; (b) (i) H2, (ii) K2CO3; (c) BnO2CCl or EtO2CCl; (d) MOM-Cl; (e) LiBH4; (f) Ac2O.
Results and Discussion Substrate Preparation. In a previous paper, we outlined a simple and rapid method for the preparation of meso cis-2,6-piperidines.10 Unfortunately, we have since discovered that this method leads to a mixture of cis:trans isomers (∼8:1) which are inseparable by standard chromatographic techniques. This forced the development of a different route to the required meso piperidines. Though this new method is longer by one step, the overall yield of the substrates is significantly higher than for the former preparation. Esterification of pyridine-2,6-dicarboxylic acid (7) in absolute MeOH, 2,2-dimethoxypropane and 12 N HCl gave the diester hydrochloride 8 (Scheme 1). The crude pyridine salt was hydrogenated in H2O over palladium on carbon, and after filtration of the catalyst, the free piperidine 9 was generated by the addition of solid K2CO3. This amino diester was recrystallized in hexane before further use to ensure the complete absence of any contamination by the trans isomer. Protection of the amine with the appropriate carboalkoxy chloride yielded 10, which was reduced with LiBH4 to give the diol 11. Acetylation with acetic anhydride afforded the diacetate 12. The synthesis of the meso cis,cis-2,4,6-piperidine substrates followed a route similar to that of the 2,6piperidines (Scheme 2). Esterification of chelidamic acid (13) under the same conditions as for 7 produced the
J. Org. Chem., Vol. 61, No. 10, 1996 3333
diester 14. Hydrogenation of the crude amine hydrochloride in H2O over rhodium on alumina11 followed by liberation of the amine with K2CO3 gave the amino alcohol 15 which was recrystallized before further use to ensure cis,cis purity. The yield of 15 was moderate (54% from 13) because of losses due to hydrogenolysis of the C-O bond at position 4, but no attempt was made to improve the yield, since Dreiding’s study reported that the above hydrogenation conditions were the best possible.11 Protection of the amine with the appropriate carboalkoxy chloride gave 16, and protection of the alcohol as the MOM ether yielded the diester 17. Reduction of this diester with LiBH4 gave the diol 18 which was acetylated to afford the diacetate 19. Enzymatic Desymmetrization. Initially, a wide range of lipases and proteases were screened for activity with the meso cis-piperidine substrates. All tested enzymes in a variety of aqueous reaction conditions showed no or very little hydrolytic activity in the presence of diesters 10 and 17. The enzymatic acetylation of diols 11 and 18 in organic solvent was more successful, but poor yields of monoacetate and competitive conversion to diacetate rendered this technique of little value. However, the reverse reaction, enzymatic hydrolysis of the diacetates 12 and 19, gave good to excellent results in terms of both chemical yield and enantioselectivity (Table 1). Three enzymes consistently hydrolyzed the four acetates. Pig liver esterase (PLE) hydrolyzed the acetates relatively quickly, but the chemical yields of monoacetates 20 and 21 were negligible to poor. Hydrolysis in the presence of wheat germ lipase (WGL) provided monoacetates of good enantiomeric purity (7193%) but only moderate chemical yield (30-58%). Aspergillus niger lipase (ANL) in the presence of 7% CH3CN gave very high enantiomeric excess values (ee g 98%) and good to excellent chemical yields (76-92%). The first set of experiments with ANL were conducted in pure phosphate buffer at pH 7. However, within 1824 h the growth of a bacillus in the medium (observed by microscopic analysis) made the reaction difficult to monitor and necessitated the addition of more enzyme to complete the reaction. It was found that by adding 7% CH3CN to the reaction, the growth of the bacillus (probably a spore contaminant in the bought enzyme) was suppressed. This had the effect of slowing the reaction, but both chemical yield and enantioselectivity were improved. The enantiopurities of all the monoacetates were determined by formation of Mosher’s ester and analysis of the diastereomeric composition with 19F NMR. Absolute Configuration. Since the absolute configuration of the monoacetate 20a produced by the action of ANL on 12a is known,10 the absolute configuration of 20b from ANL hydrolysis was not difficult to ascertain. The N-carbobenzoxy monoacetate 20a was mesylated under standard conditions to give the unstable acetate 22a (Scheme 3). Addition of 0.5 N NaOH to a solution of 22a in THF and MeOH hydrolyzed the acetate and provoked concomitant, intramolecular attack on the carbonyl of the carbamate by the newly formed hydroxy group to form the oxazolidinone 23 in 73% yield. The moderate yield of 23 was due to an interesting secondary reaction. The alcohol was found to also attack the methylene carbon of the 2-substituent, thereby displacing the mesylate to give a meso N-carbobenzoxy bicyclic ether in 20% yield. This sequence of reactions was repeated (11) Dreiding, A. S.; Hermann, K. Helv. Chim. Acta 1976, 59, 626.
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Table 1. Enzymatic Desymmetrization of Diacetates 12 and 19
time (h) (1 equiv, hydrolysis) 12 enzyme PLE WGL ANL ANL (7% CH3CN)
yield (% monoOAc)
19
20
21
20
a b a b a b a b R ) Bn R ) Et R ) Bn R ) Et R ) Bn R ) Et R ) Bn R ) Et 1.5 40 50 108
3 9 24 72
2 31 46 102
16 24 39 95
